1. Field of the Invention
The present invention relates to a gear stage detection device, and more particularly to a device for indirectly detecting a gear stage and gear position of a vehicle transmission without recourse to a gear position sensor.
2. Description of the Related Art
The present inventors have newly developed a power transmission device for a vehicle in which a fluid coupling (including a torque converter) which may be locked up and a friction type gear shift clutch are provided in series between an engine and a transmission such that the speed shift clutch is engaged and disengaged automatically during a gear shift. In this case, during a gear shift the clutch is automatically disengaged at the beginning of gear disengagement, and the clutch is automatically engaged when the gear is engaged.
Since clutch control is performed in accordance with the gear stage position of the transmission in this manner, and also since the method of controlling clutch engagement and disengagement (snore specifically the engagement/disengagement speed, engagement/disengagement amount, and so on) may change at each gear stage, a gear position sensor is provided in the transmission for detecting the gear stage. When the gear stage is detected by the gear position sensor alone, however, problems occur such as clutch control becoming impossible when the sensor is broken, and hence this is not a failsafe solution. As a backup thereto, a method is disclosed in Japanese Patent Application Laid-Open Publication No. H4-171353 for determining the current gear stage using the outputs of a vehicle speed sensor and an input shaft rotation sensor.
According to the technology disclosed in this publication, however, a predetermined calculation is performed by converting the output of each sensor into a rotational frequency per time unit, or in other words a rotation speed. Although this is not problematic at comparatively high vehicle speeds, when the vehicle speed is comparatively low similar values are obtained for the rotational frequency of the transmission, causing the margin of error between the actual value and the calculated value to increase such that misjudgments of the gear stage may occur.